1. Field of the Invention
The present invention relates to an improvement on the tread pattern of a pneumatic tire, and more particularly to a pneumatic tire for use in construction or industrial vehicles that has a tread pattern in which uneven wear resistance can be enhanced without causing degradation in the various characteristics of the tire.
2. Description of the Related Art
In general, in a pneumatic radial tire for use in construction or industrial vehicles, as shown in FIG. 3, main lug grooves 2 are disposed in the opposing shoulder regions of a tread portion 1 at a predetermined pitch in the circumferential direction of the tire. The main lug grooves 2 are so arranged as to provide circumferential phase difference between the opposing tread shoulder regions.
Such a lug-type tread pattern is excellent in driving force, braking force, and particularly tractive force when operated on an unpaved road. Therefore, it is customarily employed as a tread pattern in a pneumatic tire for use in construction and industrial vehicles.
In a pneumatic tire having a conventional tread of a lug-type pattern, however, the opposing tread shoulder portions tend to wear more rapidly than the tread center portion, which results in occurrence of uneven wear. There has thus far been found no satisfactory solution to prevent such occurrence of uneven wear.
In view of the foregoing problems, an object of the present invention is to provide a pneumatic tire having a tread pattern in which occurrence of uneven wear as seen in a conventional lug-type tread pattern can be prevented without causing degradation in the various characteristics of the tire.
To solve the above-described problem, the present inventor carried out extensive research of a lug-type tread pattern and finally found that the above-mentioned object can be achieved by disposing a narrow shallow groove in the central region of the tread portion in its width direction for connecting the main lug grooves located in the opposing tread shoulder regions, and by forming a shallow groove portion in the shoulder end region of the main lug groove. The present invention has been accomplished based on this novel finding.
That is, according to the present invention, there is provided a pneumatic tire having a tread pattern in which main lug grooves are disposed in the opposing tread shoulder regions at a predetermined pitch in the circumferential direction of the tire, the main lug grooves being so arranged as to provide circumferential phase difference between the opposing regions. In the pneumatic tire, a narrow shallow groove is disposed in the central region of the tread portion in its width direction for connecting the main lug grooves located in the opposing tread shoulder regions, and a shallow groove portion is formed in the shoulder end region inside the main lug groove.
In the construction of the present invention, it is preferable that the groove depth of the narrow shallow groove be set in a range of 15 to 30% of the groove depth of the main lug groove, that the region in which the narrow shallow groove is arranged be set in a range of 20 to 40% of the tread width, and that the groove width of the narrow shallow groove be set in a range of 35 to 100% of the groove width of the main lug groove. Moreover, it is preferable that the groove depth of the shallow groove portion inside the main lug groove be set in a range of 50 to 80% of the groove depth of the main lug groove, and that the region in which the shallow groove portion is formed inside the main lug groove be set in a range of 20 to 50% of the groove length of the main lug groove extending from the tread end to the tread center.
In a pneumatic tire having a lug-type tread pattern, the tread shoulder region tends to wear more rapidly than the tread center region. This is due to the input difference of inputs between the shoulder and center portions, specifically, the input value for the shoulder portion is found to be larger than that for the center portion. Accordingly, in the present invention, in order to the wear to be distributed uniformly, the circumferential rigidities of the tread center and shoulder portions are optimized, that is, the tread center portion is made to have lower rigidity, and, in contrast, the tread shoulder portion is made to have higher rigidity. In this way, it is possible to successfully prevent occurrence of uneven wear as seen in a conventional lug-type tread pattern, which has thus far been difficult to be prevented sufficiently.